1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and, more specifically for the catalytic isomerization of olefinic hydrocarbons.
2. General Background
Olefinic hydrocarbons are feedstocks for a variety of commercially important addition reactions to yield fuels, polymers, oxygenates and other chemical products. The specific olefin isomer, considering the degree of branching or position of the double bond, may be important to the efficiency of the chemical reaction or to the properties of the product. In plasticizer applications, for example, the more-linear (less-branched) isomers are more efficient in that a lower concentration is required to impart a specified softness to polyvinyl chloride.
Olefin isomers rarely are obtained in a refinery or petrochemical product in a ratio matching product demand. For example, there is a widespread need to increase the proportion of isobutene, isopentene and other tertiary-carbon olefins for production of MTBE, TAME and other ethers. In contrast, oligomerization of lower olefins produces an olefinic product which has an excessive proportion of alkyl substituents for high-quality plasticizer production. Catalytic isomerization to alter the ratio of isomers is one solution to these needs. Since isomerization competes with alternative feedstock sources as a source of desired isomers, an isomerization process must be efficient and relatively inexpensive. The principal problem facing workers in the art therefore is to isomerize olefins to increase the concentration of the desired isomer while minimizing product losses to heavier or lighter products.
3. Related Art
Processes for the isomerization of olefinic hydrocarbons are widely known in the art. Many of these use catalysts comprising phosphate. U.S. Pat. No. 2,537,283 (Schaad), for example, teaches an isomerization process using an ammonium phosphate catalyst and discloses examples of butene and pentene isomerization. U.S. Pat. No. 3,211,801 (Holm et al.) discloses a method of preparing a catalyst comprising precipitated aluminum phosphate within a silica gel network and the use of this catalyst in the isomerization of butene-1 to butene-2. U.S. Pat. Nos. 3,270,085 and 3,327,014 (Noddings et al.) teach an olefin isomerization process using a chromium-nickel phosphate catalyst, effective for isomerizing 1-butene and higher alpha-olefins. U.S. Pat. No. 3,304,343 (Mitsutani) reveals a process for double-bond transfer based on a catalyst of solid phosphoric acid on silica, and demonstrates effective results in isomerizing 1-butene to 2-butenes. U.S. Pat. No. 3,448,164 (Holm et al.) teaches skeletal isomerization of olefins to yield branched isomers using a catalyst containing aluminum phosphate and titanium compounds. U.S. Pat. No. 4,593,146 teaches isomerization of an aliphatic olefin, preferably 1-butene, with a catalyst consisting essentially of chromium and amorphous aluminum phosphate. None of the above references disclose the olefin-isomerization process using the non-zeolitic molecular sieve (NZMS) of the present invention.
The art also contains references to the related use of zeolitic molecular sieves. U.S. Pat. No. 3,723,564 (Tidwell et al.) teaches the isomerization of 1-butene to 2-butene using a zeolitic molecular sieve. U.S. Pat. No. 3,751,502 (Hayes et al.) discloses the isomerization of mono-olefins based on a catalyst comprising crystalline aluminosilicate in an alumina carrier with platinum-group and Group IV-A metallic components. U.S. Pat. No. 3,800,003 (Sobel) discloses the employment of a zeolite catalyst for butene isomerization. U.S. Pat. No. 3,972,832 (Butler et al.) teaches the use of a phosphorus-containing zeolite, in which the phosphorus has not been substituted for silicon or aluminum in the framework, for butene conversion. None of the above teach the use of NZMS for the present isomerization process, and Butler et al. discloses high yields of heavier olefins from butenes at a range of temperatures with a phosphorus-containing zeolite.
U.S. Pat. No. 4,503,282 (Sikkenga) reveals a process for converting linear alkenes to isomerized alkenes using a crystalline borosilicate molecular sieve, with examples demonstrating the conversion of linear butenes to isobutene. U.S. Pat. No. 5,132,467 (Haag et al.), filed Mar. 6, 1991, teaches a combination of two-stage etherification followed by common fractionation and olefin isomerization; the isomerization is carried out over a medium-pore metallosilicate catalyst with a range of ZSMs and MCM-22 being disclosed. The isomerization of olefins using NZMS, containing tetrahedral aluminum, phosphorus and at least one other element, has not been disclosed in the above references.
U.S. Pat. No. 5,107,050 (Gaffney et al.), filed Dec. 28, 1990, discloses butene isomerization using a MgAPSO or SAPO molecular sieve at a temperature above 900.degree. F. U.S. Pat. No. 5,136,108 (Gaffney et al.), filed Mar. 6, 1991, teaches a combination process for producing TAME and/or TAA by reacting tertiary pentenes with methanol and/or water, distillation to separate reactants, and isomerization of C.sub.5 hydrocarbons with return of branched hydrocarbons to TAME/TAA production; preferred isomerization catalysts are SAPOs and MgAPSOs.
"Non-zeolitic molecular sieves" or "NZMSs" as referenced herein include the "SAPO" silicoaluminophosphates of U.S. Pat. No. 4,440,871 (Lok et al.), the "FAPO" ferroaluminophosphates of U.S. Pat. No. 4,554,143 (Messina et al.), and the metal aluminophosphates of U.S. Pat. No. 4,567,029 (Wilson et al.) wherein the metal is at least one of Mn, Co, Zn and Mg. The application of NZMS-containing catalyst to the isomerization of a C.sub.8 aromatics stream is revealed in U.S. Pat. No. 4,740,650 (Pellet et al.). U.S. Pat. No. 4,689,138 teaches a process for isomerizing normal and slightly branched paraffins using a catalyst comprising SAPO molecular sieves. The use of MgAPSO compositions for hydrocarbon conversion is taught in U.S. Pat. No. 4,882,038. However, none of these references discloses or suggests the present olefin-isomerization process.